The Seebeck effect, or the thermoelectric effect, is the voltage difference that exists between two points of a material when a temperature gradient is established between those points. Materials, usually semiconductors or conductors, which exhibit this phenomenon, are known as thermoelectrics or thermoelectric materials. Devices made from thermoelectric materials take advantage of the Seebeck effect to convert heat into electricity. For instance, the Seebeck effect is the physical basis for a thermocouple, which is often used in temperature measurement.
Measurements of the Seebeck effect are reported as the Seebeck coefficient (S) in units of μV/K (microvolts per Kelvin). The Seebeck coefficient can be defined as the ratio between the open circuit voltage and the temperature difference, between two points on a conductor, when a temperature difference exists between those points. The Seebeck coefficient can take either positive or negative values depending upon whether the charge carriers are holes or electrons respectively.
The efficiency of thermoelectric materials is a monotonically increasing function of the figure-of-merit, Z=S2σ/κ, where σ (in units of Ω−1cm−1) is the electrical conductivity, and κ (in units of W/cm K) is the thermal conductivity. In determining device efficiency, Z times the temperature (ZT) is a useful metric and is dimensionless. A material needs a large absolute S to maximize ZT, while electrical resistivity and thermal conductivity should be low. A high electrical conductivity results in minimizing Joule heating in the thermoelectric material, while a low thermal conductivity helps to maintain large temperature gradients in the material. Another useful metric is the power factor which is simply the square of the thermopower times the electrical conductivity.
Metals and metal alloys received much interest in the early development of thermoelectric applications, but these materials have a high thermal conductivity. Furthermore, the Seebeck coefficient of most metals is on the order of 10 μV/K, or less. Depending upon the doping level semiconducting materials can attain Seebeck coefficients greater than 100 μV/K. Generally, semiconductors can also possess moderately high electrical conductivity and low thermal conductivity, which further increases Z, and thus the efficiency of the thermoelectric material. For instance, bismuth telluride (Bi2Te3) and lead telluride (PbTe) are two commonly used semiconductor thermoelectric materials with optimized ZT close to 1. Bismuth telluride's optimal operating temperature is around 300 K and PbTe is around 700 K. Optimized materials are complex alloy compositions, such as Sb1.6Bi0.4Te, Bi2Te2.4Se0.6, or Pb0.6Sn0.4Te, with various dopants to control thermal conductivity and carrier concentration. No commercially available materials exist with ZT substantially greater than 1. As shown in FIG. 1, no commercial materials exist with ZT of 1.5 or greater. A material possessing higher ZT is more efficient and a ZT of ˜4 would be required to approach the thermodynamic efficiencies obtained by conventional internal combustion engines.
As mentioned above, optimizing the ZT of a material generally involves synthetic methods by which the stoichiometry of the starting material is altered by doping and/or by alloying with aliovalent elements. Dopants are generally intended to increase the electrical conductivity of the material, while alloying is intended to reduce the thermal conductivity and modify the band gap. Often, this leads to a material with an entirely different composition from the parent compound. However, in many materials dopants are not electrically active, due to the presence of compensating defects that are induced or modified by doping. Consequently, there is no easy way to predict the Seebeck coefficient of the resulting material composition, which can be diminished due to doping and alloying.